At about 16 weeks of pregnancy, the retina, at the back of the inside of the eye, begins to develop blood vessels which provide nourishment and oxygen to the eye. Over time, this network of vessels gradually grows forward, eventually covering the entire surface of the retina. The process is usually complete at the end of term, or about 40 weeks gestation.
Prematurity can interfere with the development of these blood vessels. This condition is called retinopathy of prematurity (ROP) and it affects many premature babies. As with other conditions, the risk of developing ROP increases with the degree of prematurity. For example, about 80% of extremely premature babies, those of less than 26 weeks gestational age, have ROP whereas only about 15% of mildly premature babies, more than 30 weeks gestational age, do. The babies at highest risk for ROP include those who are born very prematurely or those with very low birth weight of 1500 g or less.
Although most cases are mild and treatable, ROP can permanently affect vision. Depending on the severity, ROP may cause minor vision impairments or, in more rare cases, blindness, especially if the condition goes untreated.
The blood vessels generally grow from areas of high oxygenation, where the vessels already exist, to areas of low oxygenation, where the vessels have not yet formed. In this way, the retina is gradually and evenly covered in blood vessels that grow from the back of the eye towards the front like a wave approaching the shoreline.
The oxygen level of the baby’s environment is an important factor for the progress of normal blood vessel development. When the baby is in the womb, the concentration of oxygen in its blood stream is kept relatively steady and is lower than the levels that will occur after birth. In comparison, after a premature baby is born, oxygen levels in the bloodstream are kept in the desired range by adding oxygen to the air surrounding the baby; this helps the premature baby’s breathing. This rapid change in the level of oxygen in the baby’s bloodstream has an important effect on the regulation of blood vessel development. Moreover, if the baby is having other complications that affect the regular flow of oxygenated blood, this too can interfere with the normal growth of the blood vessels.
There are many factors that contribute to the development of ROP. The knowledge that oxygen levels have an effect came in the 1940s. At that time, many premature babies were given supplemental oxygen whether they were having breathing problems or not. At this time it was discovered that the babies who were given high concentrations of oxygen had a much higher incidence of ROP than babies who were not given oxygen. Although supplemental oxygen is often essential for a premature baby’s survival, especially those with the more severe form of lung and breathing complications, the level of supplemental oxygen must be carefully monitored and adjusted to keep oxygen levels in the bloodstream in the desired range. Following this adjustment, the frequency of ROP has gone down but not disappeared.
Stages of ROP
ROP is graded, or classified into stages of severity, from Stage 1, the mildest case, to Stage 5, the most severe. At what stage ROP is graded depends not only on the extent of the abnormal blood vessel growth but where in the eye the growth has taken place.
Fortunately, most cases of ROP are mild and resolve by themselves. However, when ROP is severe and does not resolve with maturation of the baby, if left untreated these abnormal blood vessels can continue to grow on the surface of the retina and result in formation of fibrous tissue. This in turn can pull on the retina and may lead to partial or full detachment of the retina.
All premature babies are screened for ROP. If a problem is identified, screening will continue at regular intervals. If treatment is deemed necessary, surgery, usually with laser photocoagulation or cryotherapy, a kind of freezing, is performed.